Credit: Slug Signorino

I came across your column on what zero means on the Fahrenheit calibration. Y'all blew it. You said that, unlike 32 or 212 degrees, zero degrees corresponded to nothing in nature—information technology was merely an arbitrarily assigned number. It isn't. It's the temperature at which seawater will freeze. Of course it's an approximation, because the freezing point of saltwater varies based on salinity, but nothing degrees is a dominion of thumb. I'm non playing gotcha hither—just battling misinformation wherever information technology rears its ugly head. —Richard Forte

Then allow me assist yous in your battle, Richard: You're wrong.

I admit yous've got a lot of company. Wikipedia takes your side, as does at least one college physics textbook. Simply close exam makes it reasonably clear the seawater explanation derives from a misreading of the testify.

In my 1989 column I said that Daniel Gabriel Fahrenheit based his system of temperature measurement on an earlier calibration devised by Danish astronomer Ole Roemer. Roemer, I said, had set nil arbitrarily—his principal consideration was that it was colder than the temperature ever got in Denmark, because he didn't like using negative numbers in his weather logbook.

Roemer's scale had 7.v as the freezing point of water and 22.5 as body temperature, in those days chosen "claret rut." To get rid of the awkward fractions, Fahrenheit did some multiplication, somewhen winding up with 32 as the freezing point and 96 as body temperature. (Boiling betoken initially didn't figure in his scheme.)

I said that when Fahrenheit was set up to demonstrate his arrangement to London's Purple Society in 1724, he worried it would look odd if nil on his scale was untethered to reality, and thus had to concoct a rationale. Here's what he wrote in the paper he presented:

"The division of the scale depends on iii fixed points, which tin be determined in the following style. The get-go is constitute in the uncalibrated office or the first of the scale, and is determined by a mixture of water ice, water and sal ammoniac [ammonium chloride], or even body of water salt." The "or even" part (the Latin phrase is vel etiam [salis] maritimi) is a giveaway—the freezing point of seawater was an reconsideration. Fahrenheit underscores this equally he continues: "If the thermometer is placed in [the water-ice-ammonium chloride] mixture, its liquid descends as far as the degree that is marked with a nada. This experiment succeeds meliorate in winter than in summertime."

Call up what this means: The method supposedly used to make up one's mind zero on Fahrenheit's scale doesn't always work. Who would be foolish enough to invent a temperature calibration that wouldn't permit thermometers to be reliably calibrated? In contrast, the freezing indicate of fresh h2o, every bit manifested in an ice/water mixture, is constant for practical purposes, making information technology a dependable benchmark. Information technology seems obvious the ammonium chloride/seawater procedure had been invented after the fact to provide a physical correlative for a point originally called for other reasons.

But don't take my discussion for it. In a letter Fahrenheit wrote on April 17, 1729, he says that when he visited Roemer in 1708, he found several thermometers beingness calibrated past standing in water and ice. These thermometers were so heated to torso heat, and "after [Roemer] had marked these two points on them all, half the distance found between them was added below the point of h2o and ice, and this whole distance was divided into 22.5 parts, kickoff at the lesser with 0, arriving thus at 7.5 for the point of water mixed with ice, and 22.5 for the signal of blood heat."

At that place you lot have it. Fahrenheit, following Roemer, but determined the altitude betwixt the marks for the freezing point of water and body heat on his glass thermometers (64 degrees, in the scale he would ultimately develop), measured off half this distance (32 degrees) below the freezing indicate, and called that zilch.

Recounting this story in a 1991 article, R.J. Soulen of the U.S. Naval Inquiry Laboratory writes: "The zip on this calibration had no fundamental meaning, post-obit the tradition of others. Fahrenheit chose to ascertain a nix below the coldest temperature likely to exist encountered by everyday use of his thermometers."

As I said. To be fair, Fahrenheit wasn't the only early scientist to come with quirky calibration procedures: Robert Boyle proposed that thermometers be calibrated to the temperature of congealing aniseed oil; Joachim Dalencé suggested pegging them to the freezing point of water and the melting point of butter; and the Encyclopaedia Britannica thought a useful temperature reference point was "water just hot enough to permit wax, that swims upon information technology, brainstorm to coagulate."

At least these benchmarks were practical. Effort calibrating your thermometer using the standard proposed by 19th-century astronomer Charles Piazzi Smith, who nominated a scale set to "the mean temperature of the King's Chamber at the centre of the Great Pyramid of Giza." Great idea, Chuck. —Cecil Adams